applyCumulative method on Aggregator
test for applyCumulative
use CanBuildFrom in applyCumulative
use TraversableOnce not TraversableLike
separate out cumulativeIterator from applyCumulatively
remove duplicate definition of applyCumulative
Merge pull request #386 from avibryant/avi-cumulative

applyCumulative method on Aggregator
Add Aggregator.zip

This returns a composite aggregator with a tuple input, which comes in
handy if you have an initial input that has to be prepared differently
for each of the joined aggregators.